Package net.schmizz.sshj.transport.mac

Interface MAC

All Known Implementing Classes:

BaseMAC, HMACMD5, HMACMD596, HMACRIPEMD160, HMACSHA1, HMACSHA196, HMACSHA2256, HMACSHA2512

public interface MAC

Message Authentication Code for use in SSH. It usually wraps a javax.crypto.Mac class.

Method Summary

Modifier and Type	Method	Description
byte[]	doFinal()
byte[]	doFinal(byte[] input)
void	doFinal(byte[] buf, int offset)
int	getBlockSize()
void	init(byte[] key)
boolean	isEtm()	Indicates that an Encrypt-Then-Mac algorithm was selected.
void	update(byte[] foo)
void	update(byte[] foo, int start, int len)
void	update(long foo)

Method Detail

doFinal

byte[] doFinal()

doFinal

byte[] doFinal(byte[] input)

doFinal

void doFinal(byte[] buf,
             int offset)

getBlockSize

int getBlockSize()

init

void init(byte[] key)

update

void update(byte[] foo)

update

void update(byte[] foo,
            int start,
            int len)

update

void update(long foo)

isEtm

boolean isEtm()

Indicates that an Encrypt-Then-Mac algorithm was selected.

This has the following implementation details. 1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms

OpenSSH supports MAC algorithms, whose names contain "-etm", that perform the calculations in a different order to that defined in RFC 4253. These variants use the so-called "encrypt then MAC" ordering, calculating the MAC over the packet ciphertext rather than the plaintext. This ordering closes a security flaw in the SSH transport protocol, where decryption of unauthenticated ciphertext provided a "decryption oracle" that could, in conjunction with cipher flaws, reveal session plaintext.

Specifically, the "-etm" MAC algorithms modify the transport protocol to calculate the MAC over the packet ciphertext and to send the packet length unencrypted. This is necessary for the transport to obtain the length of the packet and location of the MAC tag so that it may be verified without decrypting unauthenticated data.

As such, the MAC covers:

mac = MAC(key, sequence_number || packet_length || encrypted_packet)

where "packet_length" is encoded as a uint32 and "encrypted_packet" contains:

byte padding_length byte[n1] payload; n1 = packet_length - padding_length - 1 byte[n2] random padding; n2 = padding_length

Returns:

Whether the MAC algorithm is an Encrypt-Then-Mac algorithm